Transferrin, the plasma protein responsible for the transport of iron in the circulation and its delivery to cells requiring this essential metal for the biosynthesis of iron-proteins, has two distinguishable binding sites per molecules. Using techniques developed and documented in our laboratory transferrin labeled with iron at either of these two sites will be prepared and used to determine, as definitively as possible, whether the sites are functionally equivalent in their physiologic iron-donating properties. Concurrently, ENDOR studies of transferrin will be continued to explore the effects of different metal ions and different anions on the spectroscopic properties of the sites. Physicochemical characteristics of the transferrin receptor of erythroid cells are being measured in an effort to estimate its molecular size and subunit structure, and to explore the chemistry of the receptor-transferrin interaction. Using the Friend erythroleukemic cell, we also plan to study the rate of synthesis of transferrin receptors.